Robotic arms have been utilized in a wide range of industries thanks to certain characteristics thereof such as high durability, automatic control, and high precision. In compliance control of a robotic arm, the robotic arm is subjected to an external load during operation (operated alone or by an operator in a man-machine operation), and some parameters associated with an output of the robotic arm need to be adjusted according to the external load in order to output a torque that can achieve gravity compensation, thereby ensuring normal operation under the external load.
Achieving the effect of gravity compensation in response to a change in subjected load applied to the robotic arm involves obtaining an amount of the change in the subjected load in advance. One way to do so is to measure a weight of the external load and input a measured weight to the robotic arm before the external load is applied to the robotic arm. In this way, a processor of the robotic arm is able to calculate the parameters to be adjusted based on the measured weight.
Another way involves installing a torque sensor on each joint of the robotic arm and, after the external load is applied to the robotic arm, calculating a change in a torque sensed by each torque sensor to obtain the amount of the change in the subjected load of the robotic arm in comparison to a previously applied load. U.S. Pat. No. 9,533,414 B2 discloses a torque detecting method for an arm device. In the torque detecting method, for the external load applied to the arm device, a mechanical deviation is detected, and a gravity coefficient corresponding with a specific rotational angle of the arm device is calculated. Then, a gravitational torque based on a position of the arm device and the gravity coefficient is calculated. Afterward, using a calibration process, an actual output torque is calculated based on an operating torque detected by the torque sensor and the gravitational torque.